Okay, now picture him with the lower half of his body as that of a fish. Now, think horns.

That's Oceanus, or Ogenos.

Oceanus, sometimes called simply Ocean, is one of the titans in exile, defrocked from his throne as ruler over the seas by Poseidon. Before the uprising of the Olympian Gods, Oceanus ruled the seas, rivers, lakes, ponds, and other bodies of water with his wife and sister, Tethys. They were the offspring of Gaia (earth) and Ouranos (sky). Their offspring included ocean nymphs, called Oceanids, and there were 3,000 of them. Their sons, were water spirits, and had horns and tails like their father. They also numbered 3,000.

Poor Tethys.

In some stories, Ocean is the personification of the great sea that encircles the earth. The earth was thought to be a large disk located in the center of the great Ocean. In others, Tethys is the personification of the sea and Ocean rules over her. In either case, the ocean was the source of all fresh waters, such as rain, rivers and aquifers of the earth.